Programmable dc power supply

ABSTRACT

A programmable DC power supply including a proportionally controlled transistor connected in series with the load for varying the load current in accordance with the magnitude of a control signal applied to the transistor. A detector coupled across the transistor functions to sense high and low voltage thresholds and thereupon select an incrementally higher or lower voltage for application to the load to assure that the transistor operates continuously in its proportional range, thereby enabling the input signal to maintain control of the load current.

United States Patent Carpentier et al.

PROGRAMMABLE DC POWER SUPPLY lnventors: Richard A. Carpentier; SpencerA.

Shriver; Richard C. Smith, all of Charlottesville, Va.

Assignee: Sperry Rand Corporation Filed: Sept. 8, 1970 Appl. No.: 70,259

US. Cl. ..323/22 T, 321/18, 32 l/45 R, 323/25, 323/38 Int. Cl. ..G05f1/56 Field of Search ..323/4, 9, 22 T, 22 SC, 22 R, 323/24, 25, 38;321/18, 24, 45 R; 307/207, 247

References Cited UNITED STATES PATENTS 4/1970 Thomas "3 21/18 Mar. 14,1972 Johnston ..323/22 SC Moyer et al. ..32 l/45 DR X PrimaryEtaminer-Gerald Goldberg Attomey--S. C. Yeaton [57] ABSTRACT Aprogrammable DC power supply including a proportionally controlledtransistor connected in series with the load for varying the loadcurrent in accordance with the magnitude of a control signal applied tothe transistor. A detector coupled across the transistor functions tosense high and low voltage thresholds and thereupon select anincrementally higher or lower voltage for application to the load toassure that the transistor operates continuously in its proportionalrange, thereby enabling the input signal to maintain control of the loadcurrent.

12 Claims, 3 Drawing Figures lNPUT r imam PROPORTION/EL AMPLIFIER DRIVERR5 NONINVERTING 15 PROPORTIONAL AMPLIFIER DRIVER Rel l R4 R9 lBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to power supplies and more particularly to aprogrammable DC supply for providing controlled direct current to avariable-load circuit.

' 2. Description of the Prior Art In certain applications, for instance,degaussing large objects such as ships or other craft, a variable directcurrent source must be provided which is capable of supplying to avariable load a comparatively large current extending over a wide rangeof values with high dynamic accuracy. One way to accomplish this, ofcourse, is to provide a constant high level voltage that is capable ofsupplying any current up to the desired maximum. However, when it isdesired to control the current level over a range of values which theload may assume during normal operation, this approach is not completelysatisfactory for the reason that intolerable power dissipation willoccur in the control device under conditions of low load impedance orhigh current flow. This problem can be circumvented by using parallelconnected control devices but only with anattendant increase in cost,size and weight and a con- -comitant reduction of efficiency andreliability. A further disadvantage of simply supplying a constant highlevel voltage is that the current cannot be controlled over large ratiosof values, say 100 to 1.

Another method of control is to switch pulses of current for variabletime intervals to vary the output current. This requires a switchingrate fast enough to accommodate the dynamics of the supply and filteringto remove the undesired alternating current components of the pulses. Inorder to provide high dynamic performance from a 60 Hz. power source itis necessary to rectify, filter, switch and filter again. The bulk andcomplexity of such a system'make it undesirable.

SUMMARY OF THE INVENTION The aforementioned disadvantages andlimitations are overcome with the present invention by the provision ofmeans for detecting high and low voltage thresholds across the controldevice and then selecting a higher or lower voltage for appli cation tothe load. More specifically, a monopolar embodiment or the inventionutilizes a proportionally controlled transistor connected in series withthe load. An input signal for controlling the level of theunidirectional DC load current is applied to the base of the transistorto vary the collector to emitter impedance thereof accordingly. For acondition where the load impedance is increasing and/or the controlsignal is calling for more current, the voltage drop across the loadincreases with an accompanying decrease of the voltage across thecontrol transistor. Ultimately this would result in saturation of thetransistor and then the input signal would no longer control the loadcurrent. At a level slightly above the transistor saturation voltage,however, a higher voltage is selected for application to the seriescombination of the load and transistor thereby assuring continuousoperation of the transistor in its proportional range. Likewise, if theload impedance is decreasing and/or the control signal is calling forless current, the voltage across the load will decrease while the dropacross the transistor simultaneously increases. Again, at apredetermined level a different voltage is selected for application tothe load and transistor, except that in this instance the voltage isdecreased from the previous value, thus assuring that the voltageacrossthe transistor does not become excessIve.

In another embodiment of the invention, wherein a capability isestablished for reversing the direction of current flow through the loadin accordance with the polarity of the control signal, the load iscoupled between the output terminals of a bridge circuit comprising apair of proportionally controlled transistors and a pair of switchingtransistors. The current magnitude is controlled as in theaforedescribed embodiment and its direction is detennined in accordancewith the particular set of proportional and switching transistors whichare activated in response to each polarity of the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. la and 1b present a schematicillustration of a bipolar variable DC supply embodying the principles ofthe present in vention.

FIG. 2 is a schematic of the switch driver utilized in the ap paratus ofFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. la and 1b, aload 10 is connected across the output terminals C, D of power bridge11. Voltage is applied to the bridge input terminals A, B from thevoltage selection network 12. The power bridge comprises switchingtransistors Q and Q and proportionally controlled transistors Q, and 0..An input control signal is applied to the power supply at input terminal13 which connects through inverting proportional amplifier driver 14 andnoninverting proportional amplifier driver 15 to transistors Q and Qrespectively, to control the level of current supplied to the load. Theinput terminal also connects through zero detector 16 to the switchingtransistors Q and O: which function in cooperation with transistors Q,and Q, to determine the direction of current flow through the load. Thezero detector output tenninals l7 and 18 also connect to comparatorswitch 19 which operates in conjunction with comparator 20, in responseto the voltages at the collectors of proportional transistors Q, and Oto provide respective step-up and stepdown signals at predeterminedcollector voltage levels. The step-up and stepdown signals in turnoperate through logic unit 21 and switch drivers 22 to select a newvoltage level from voltage selection network 12 for application to thepower bridge input terminals A, B

The general operation of the supply is such that transistor switch Q, isdriven to the ON state, that is saturated, with 0, operating in itsproportional control range while transistors Q, and Q, are in the OFF ornonconducting state for current flow through the load in the directionfrom bridge terminal C to terminal D. For reverse current flow in thedirection from terminal D to terminal C, transistor O is driven intosaturation while transistor 0, operates in the proportional controlrange and simultaneously transistors Q and Q are switched to the OFFstate.

Voltage selection network 12 provides a plurality of discrete voltagesat given increments, for example every 17.5 volts or 17.5, 35, 52.5, 70,etc. For a zero-volt signal applied to input control terminal 13 thevoltage selection network provides the lowest voltage increment, 17.5volts, for application to the power bridge by way of lead 24. As theinput control signal increases either positively or negatively, theimpedance of the operative proportional transistor decreases calling formore current to be supplied to the load and thereby reducing the voltageat the collector of the active proportional transistor. At apredetermined lower limit slightly above the saturation voltage of theproportional transistor, the voltage thereacross becomes operativethrough comparator switch 19 and comparator 20 to select the next highervoltage, 35 volts, for application to the power bridge whereby therequired current is readily supplied to the load with sufficient voltageat the collector of the proportional transistor to assure that itfunctions in its proportional range and the input signal retains controlof the load current. Operation continues in this manner with the voltageselection network providing successively higher voltages as the inputsignal increases to larger positive or negative values. For example,assume that 52.5 volts is applied to a 26-ohm load with a requirementfor 2 amperes of current. At the 2-ampere level the voltage drop acrossthe load would be 52 volts leaving only 0.5 volts at the collector ofthe active proportional control transistor, a level below theproportional control range. To preclude this condition the voltageapplied to the bridge is increased before the proportionally controlledtransistor voltage reaches the saturation level. Thus, the followingaction occurs as the input signal increases calling for the load currentto increase; at a load current magnitude corresponding to a value ofapproximately 2 volts at the collector of the proportionally controlledtransistor, the voltage selection network is actuated to apply the nexthigher voltage, 70 volts, to the power bridge. Under this condition,with 2 amperes supplied, 52 volts will be dropped across the load andthe remaining 18 volts applied to the proportionally controlledtransistor. In a similar manner, when the input voltage decreases tosmaller positive or negative values, the bridge voltage is reduced tosuccessively lower values each time the voltage at the collector of theproportional transistor reaches an upper predetermined limit. Concurrentwith the foregoing operation, the inverting and noninvertingproportional transistor drivers 14 and 15 cooperate with zero detector16 to appropriately actuate transistors Q Q and 0 so as to control thedirection of current flow through the load in accordance with thepolarity of the input signal.

Proceeding now to a more detailed discussion of the varioussub-circuits, the inverting proportional amplifier driver 14 comprisesan operational amplifier 25 which has its output coupled throughresistor R to the base of transistor 0,, connected in a Darlingtonconfiguration with transistor 0 The input control signal applied toinput terminal 13 couples through resistor R to the inverting inputterminal of amplifier 25. A negative feedback signal obtained from thevoltage produced across resistor R connected between the emitter oftransistor Q and terminal B of the power bridge, is also applied to theinverting input terminal of amplifier 25 by way of resistor R,. Thenoninverting proportional amplifier driver 15 is essentially the same asthe inverting proportional amplifier driver. It comprises an operationalamplifier 26 which has its output coupled through a resistor R to thebase of transistor Q, connected in a Darlington circuit with transistorQ The input control signal couples through a divider network made up ofresistors R, and R to the noninverting input terminal of amplifier 26. Anegative feedback signal derived from resistor R connected between theemitter of transistor Q, and terminal B of the power bridge, is appliedto the inverting input terminal of amplifier 26 through the dividernetwork consisting of resistors R and R Diodes D, and D connected to thebase terminals of transistors 0 and Q respectively, limit the negativevoltages applied thereto.

Zero detector 16 comprises an operational amplifier 27 which receivesthe input control signal through resistor R connected to its invertinginput terminal. The output of amplifier 27 connects directly to zerodetector output terminal 18 and through an inverting amplifiercomprising transistor 0 to zero detector output terminal 17 which inturn connects through switch driver 28 to transistor switch Q, andthrough resistor R to transistor 0,; in the comparator switch. Likewise,zero detector output terminal 18 connects through switch driver 29 totransistor switch 0;, and through resistor R to transistor Q, in thecomparator switch.

in operation of the power supply, application of a positive signal toinput terminal 13 produces a negative signal through invertingproportional amplifier driver 14 at the base of transistor Q causing itto switch to the OFF or open state while simultaneously a positivesignal is produced through noninverting proportional amplifier 15 at thebase of transistor 0, so as to drive it in the proportional controlrange. The positive polarity signal produced across resistor R providesnegative feedback by virtue of being applied to the inverting inputterminal of amplifier 26 and thus transistor Q stabilizes at the pointwhere the current through resistor R provides sufficient voltage tobalance out the positive input control signal at amplifier 26. At thesame time, the positive input signal feeds through zero detector 16 toprovide positive and negative signals respectively at zero detectoroutput terminals l7 and 18. The positive signal at terminal 17 isapplied through switch driver 28 to the base of transistor Q, driving itI open while transistor 0, is closed and transistor 0, is proportionallycontrolled.

The voltage at the collector of the active proportionally controlledtransistor O is applied to comparator switch 19 by way of lead 30connected to the junction between diode D, and resistor R connected tothe collector of transistor 0,. Likewise, the voltage at the collectorof transistor O is applied by way of lead 31 to the junction of diode Dand resistor R connected to the collector of transistor 0 The positivesignal at zero detector output terminal 17 saturates transistor Qcausing the voltage at the collector of transistor 0 to be connected byway of resistor R to ground 32. Transistor 0,, however, remains in theopen state as a result of the negative voltage applied to its baseterminal from zero detector output terminal 18. Hence, the voltageacross the active transistor Q, is applied through diode D to the inputterminal 33 of comparator 20 while simultaneously functioning to backbias diode D Comparator 20 comprises operational amplifiers 34 and 35.Amplifier 34 has its inverting input terminal connected through resistorR to comparator input 33 and through resistor R to the 8* supply, thevoltage division ratio of resistors R and R being selected so as torapidly reverse the normally positive output of amplifier 34 to anegative value at the instant the voltage at the collector of activetransistor Q reaches an upper predetermined limit, say 22 volts. Theonly restriction in the choice of this upper limit is that it must begreater than the voltage increment (l7.5 volts) provided by voltageselection network 12 but not so large as to permit excessive voltage tobe applied to the proportional control transistor. Amplifier 35 has itsnoninverting input connected through resistor R to comparator inputtemiinal 33 and through resistor R to the B supply. The voltage divisionratio of resistors R and R is selected to rapidly reverse the normallypositive output of amplifier 35 to a negative value at the instant thecollector voltage of transistor 0, diminishes to a predetermined lowerlimit slightly above its saturation voltage, say two times itssaturation voltage plus the voltage across feedback resistor R orapproximately 2 volts.

Logic unit 21 responds to the respective step-up and stepdown commandsprovided at the comparator output terminals 36 and 37 to provide signalsto the appropriate switch drivers 22 and thereby actuate the voltageselection network. The logic unit can be constructed from conventionalcircuits, for example, an up-down counter inhibited from stepping belowzero or above the highest count corresponding to the number of discretevoltages obtainable from the voltage selection network. In the case of16 discrete voltages, for instance, from 17.5 volts to 280 volts inincrements of 17.5 volts, a zero count can represent selection of 17.5volts, a one count selection of 35 volts and so on up to a count of 15representing selection of 280 volts. The inhibit features preclude thevoltage from changing abruptly from 17.5 to 280 or conversely.

In the case of a negative signal applied to input terminal 13, theoperation is essentially the same except that in this instancetransistors Q; and Q become operative to direct current through the loadin the direction opposite to that resulting from a positive inputsignal.

A schematic of the switch drivers used in the apparatus of FIG. 1 isshown in FIG. 2. Each switch driver comprises a transistor Q which hasits collector connected to the center tap of the primary winding of atransformer T A5 kHz. square wave derived from the output of amultivibrator (not shown) is applied across the primary winding byconnection to terminals 39 and 40. The transformer secondary winding iscoupled through diodes D and D to output terminal 41 while the centertap of the secondary is connected to output terminal 42, a filtercapacitor C being typically connected across the output terminals. Whenused in the apparatus of FIG. 1, for example, as switch driver 28,output terminals 41 and 42 are connected respectively to the base andemitter of transistor 0, and the zero detector output terminal 17couples to switch driver terminal 43 which in turn connects throughresistor R to the base of. transistor Q The other driver terminal 44couples to the emitter of transistor Q and is typically grounded. Inoperation of the switch driver, application of'a drive signal acrossdriver terminals 43 and 44 sufficient to saturate transistor Q providesfor power transfer of the square wave, signal at terminals 39 and 40 tooutput terminals 41 and 42 with isolation provided between the driverand output signals so that no interference occurs between signal groundand the power ground of the equipment being controlled.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withinthe purview of the appended claims without departing from the true scopeand spirit of the invention in its broader aspects.

' We claim:

1. A power source for supplying bipolar variable DC current to. a loadin accordance with the polarity and magnitude of an input control signalapplied to said source comprising 7 a, bridge circuit having inputterminals (A) and (B) for receiving the voltage applied thereto andoutput terminals (C) and (D) adapted for connection to the load, saidbridge circuit including first and second switching elements connectedrespectively between input terminal A and output terminals (C) and (D)and first and second proportionally controlled transistors connectedrespectively between input terminal (B) and output terminals (C) and(D), means for applying the input control signal to the switchingelements of said bridge in a manner to close one of said switchingelements and open the other switching element for one polarity andconversely for the other polarity of the control signal, means forapplying the input control signal to the proportionally controlledtransistors of said bridge in a manner to open circuit the one in serieswith the closed switching element and regulate the impedance of theother in accordance with the magnitude of the control signal, meansresponsive to the voltage across the active proportionally controlledtransistor for providing a step-up signal at one level of the voltagethereacross and a step down signal at another level of voltagethereacross, and

means responsive to the step-up and stepdown signals for incrementallychanging the voltage applied to the input terminals of said bridge.

2. The apparatus of claim 1 wherein said means for applying the inputcontrol signal to the proportionally controlled transistors includesinverting and noninverting driver amplifiers said inverting driveramplifier having its input connected to receive the input control signaland its output connectedv to the base of one of said proportionallycontrolled transistors, and

said noninverting driver amplifier having its input connected to receivethe input control signal and its output connected to the base of theother of said proportionally controlled transistors.

3. The apparatus of claim 2 including first resistance means coupled tosaid one proportionally controlled transistor to provide a firstfeedback signal representative of the current flowing through said loadwhen said one transistor is active,

means couplingsaid first resistance means to said inverting driveramplifier for algebraically summing therein the input control and firstfeedback signals to provide a drive signal which controls said oneproportionally controlled transistor to regulate the load current inaccordance with the magnitude of the input control signal for onepolarity thereof,

second resistance means coupled to said other proportionally controlledtransistor to provide a second feedback signal representative of thecurrent flowing through said load when said other transistor is active,and

means coupling said second resistance means to said noninverting driveramplifier for algebraically summing therein the input control and secondfeedback signals toprovide another drive signal which controls saidother proportionally controlled transistor to regulate the load currentin accordance with the magnitude of the input control signal for theopposite polarity thereof.

4. The apparatus of claim 3 wherein said means for applying the inputcontrol signal to said bridge switching elements includes a zerodetector comprising first and second signal inverting amplifier stages,

the input of said first inverting amplifier stage being connected toreceive theinput control signal,

the output of said first inverting amplifier stage being connected todrive the second switching element 'of said bridge, and

the output of said second inverting amplifier stage being connected todrive the first switching element of said bridge.

5. The apparatus of claim 4 wherein said means responsive to the voltageacross the active proportionally controlled transistor includes acomparator switch circuit comprising,

a first switch serially connected with a resistor and a diode,

a second switch serially connected with an additional resistor and anadditional diode,

said first switch being connected to be actuated by the signal at theoutput of the first inverting amplifier stage of said zero detector,

said second switch being connected to be actuated by the signal at theoutput of the second inverting amplifier stage of said zero detector,

the junction of said resistor and said diode being connected to outputterminal (D) of said bridge circuit,

the junction of said additional resistor and said additional diode beingconnected to output terminal (C) of said bridge circuit, and

the side of said diodes opposite said junctions being coupled togetherat the output of said comparator switch.

6. The apparatus of claim 5 wherein said means responsive to the voltageacross the active proportionally controlled transistor further includesa comparator comprising first and second threshold detectors each havingits input connected to the output of said comparator switch, said firstand second threshold detectors being operative to provide said step-upand stepdown signals respectively.

7. The apparatus of claim 6 wherein the first and second switchingelements of said bridge are respective transistors.

8. The apparatus of claim 3 wherein said first resistance means isserially connected from said one proportionally controlled transistor toterminal B of the bridge circuit and said second resistance means isserially connected from said other proportionally controlled transistorto terminal B of the bridge circuit.

9. The apparatus of claim 3 wherein said means for applying the inputcontrol signal to said bridge switching elements comprises a zerodetector including at least one signal inverting amplifier stageconnected to receive the input control signal,

said zero detector providing an output signal of opposite polarity tothe input control signal for application to the one bridge switchingelement in series with the active proportionally controlled transistorto open said one switching element and another output signal of the samepolarity as the input control signal for application to the other bridgeswitching element to close said other switching element and therebyenable current to flow through the load in one direction or the other inaccordance with the polarity of the input control signal.

10. The apparatus of claim 9 wherein said means responsive to thevoltage across the active proportionally controlled transistor includesa comparator switch circuit comprising a first switch serially connectedwith a resistor and a diode,

a second switch serially connected with an additional resistor and anadditional diode,

said first switch being connected to be actuated by one of the outputsignals provided at the output of the zero detector,

said second switch being connected to be actuated by the other of theoutput signals provided at the output of the zero detector,

the junction of said resistor and said diode being connected to one ofthe output terminals of the bridge circuit and the junction of saidadditional resistor and said additional diode being connected to theother output terminal of the bridge circuit such that the activeproportionally controlled transistor is disassociated with that switchof said first and second switches of the comparator switch circuit whichis concurrently actuated by an output signal from the zero detector, and

the side of said diodes opposite said junctions being coupled togetherat the output of said comparator switch.

11. The apparatus of claim 10 wherein said means responsive to thevoltage across the active proportionally controlled transistor furtherincludes a comparator comprising first and second threshold detectorseach having its input connected to the output of said comparator switch,

said first threshold detector normally providing a step-up signal ofpredetermined polarity until the signal at the comparator switch outputdecreases to a predetermined level whereupon the step-up signal reversespolarity and changes the voltage applied to the input terminals of thebridge to the next higher increment, and

said second threshold detector normally providing a stepdown signal ofpredetermined polarity until the signal at the comparator switch outputincreases to a predetermined level whereupon the stepdown signalreverses polarity and changes the voltage applied to the input terminalsof the bridge to the next lower increment.

12. The apparatus of claim 11 wherein the predetermined level of thefirst threshold detector is slightly greater than the saturation voltageof the active proportionally controlled transistor and the predeterminedlevel of the second threshold detector is greater than the difierencebetween adjacent incremental voltage levels applied to the bridgecircuit.

1. A power source for supplying bipolar variable DC current to a load in accordance with the polarity and magnitude of an input control signal applied to said source comprising a bridge circuit having input terminals (A) and (B) for receiving the voltage applied thereto and output terminals (C) and (D) adapted for connection to the load, said bridge circuit including first and second switching elements connected respectively between input terminal A and output terminals (C) and (D) and first and second proportionally controlled transistors connected respectively between input terminal (B) and output terminals (C) and (D), means for applying the input control signal to the switching elements of said bridge in a manner to close one of said switching elements and open the other switching element for one polarity and conversely for the other polarity of the control signal, means for applying the input control signal to the proportionally controlled transistors of said bridge in a manner to open circuit the one in series with the closed switching element and regulate the impedance of the other in accordance with the magnitude of the control signal, means responsive to the voltage across the active proportionally controlled transistor for providing a step-up signal at one level of the voltage thereacross and a stepdown signal at another level of voltage thereacross, and means responsive to the step-up and stepdown signals for incrementally changing the voltage applied to the input terminals of said bridge.
 2. The apparatus of claim 1 wherein said means for applying the input control signal to the proportionally controlled transistors includes inverting and noninverting driver amplifiers said inverting driver amplifier having its input connected to receive the input control signal and its output connected to the base of one of said proportionally controlled transistors, and said noninverting driver amplifier having its input connected to receive the input control signal and its output connected to the base of the other of said proportionally controlled transistors.
 3. The apparatus of claim 2 including first resistance means coupled to said one proportionally controlled transistor to provide a first feedback signal representative of the current flowing through said load when said one transistor is active, means coupling said first resistance means to said inverting driver amplifier for algebraically summing therein the input control and first feedback signals to provide a drive signal which controls said one proportionally controlled transistor to regulate the load current in accordance with the magnitude of the input control signal for one polarity thereof, second resistance means coupled to said other proportionally controlled transistor to provide a second feedback signal representative of the current flowing through said load when said other transistor is active, and means coupling said second resistance means to said noninverting driver amplifier for algebraically summing therein the input control and second feedback signals to provide another drive signal which controls said other proportionally controlled transistor to regulate the load current in accordance with the magnitude of the input control signal for the opposite polarity thereof.
 4. The apparatus of claim 3 wherein said means for applying the input control signal to said bridge switching elements includes a zero detector comprising first and second signal inverting amplifier stages, the input of said first inverting amplifier stage being connected to receive the input control signal, the output of said first inverting amplifier stage being connected to drive the second switching element of said bridge, and the output of said second inverting amplifier stage being connected to drive the first switching element of said bridge.
 5. The apparatus of claim 4 wherein said means responsive to the voltage across the active proportionally controlled transistor includes a comparator switch circuit comprising, a first switch serially connected with a resistor and a diode, a second switch serially connected with an additional resistor and an additional diode, said first switch being connected to be actuated by the signal at the output of the first inverting amplifier stage of said zero detector, said second switch being connected to be actuated by the signal at the output of the second inverting amplifier stage of said zero detector, the junction of said resistor and said diode being connected to output terminal (D) of said bridge circuit, the junction of said additional resistor and said additional diode being connected to output terminal (C) of said bridge circuit, and the side of said diodes opposite said junctions being coupled together at the output of said comparator switch.
 6. The apparatus of claim 5 wherein said means responsive to the voltage across the active proportionally controlled transistor further includes a comparator comprising first and second threshold detectors each having its input connected to the output of said comparator switch, said first and second threshold detectors being operative to provide said step-up and stepdown signals respectively.
 7. The apparatus of claim 6 wherein the first and second switching elements of said bridge are respective transistors.
 8. The apparatus of claim 3 wherein said first resistance means is serially connected from said one proportionally controlled transistor to terminal B of the bridge circuit and said second resistance means is serially connected from said other proportionally controlled transistor to terminal B of the bridge circuit.
 9. The apparatus of claim 3 wherein said means for applying the input control signal to said bridge switching elements comprises a zero detector including at least one signal inverting amplifier stage connected to receive the input control signal, said zero detector providing an output signal of opposite polarity to the input control signal for application to the one bridge switching element in series with the active proportionally controlled transistor to open said one switching element and another output signal of the same polarity as the input control signal for application to the other bridge switching element to close said other switching element and thereby enable current to flow through the load in one direction or the other in accordance with the polarity of the input control signal.
 10. The apparatus of claim 9 wherein said means responsive to the voltage across the active proportionally controlled transistor includes a comparator switch circuit comprising a first switch serially connected with a resistor and a diode, a second switch serially connected with an additional resistor and an additional diode, said first switch being connected to be actuated by one of the output signals provided at the output of the zero detector, said second switch being connected to be actuated by the other of the output signals provided at the output of the zero detector, the junction of said resistor and said diode being connected to one of the output terminals of the bridge circuit and the junction of said additional resistor and said additional diode being connected to the other output terminal of the bridge circuit such that the active proportionally controlled transistor is disassociated with that switch of said first and second switches of the compArator switch circuit which is concurrently actuated by an output signal from the zero detector, and the side of said diodes opposite said junctions being coupled together at the output of said comparator switch.
 11. The apparatus of claim 10 wherein said means responsive to the voltage across the active proportionally controlled transistor further includes a comparator comprising first and second threshold detectors each having its input connected to the output of said comparator switch, said first threshold detector normally providing a step-up signal of predetermined polarity until the signal at the comparator switch output decreases to a predetermined level whereupon the step-up signal reverses polarity and changes the voltage applied to the input terminals of the bridge to the next higher increment, and said second threshold detector normally providing a stepdown signal of predetermined polarity until the signal at the comparator switch output increases to a predetermined level whereupon the stepdown signal reverses polarity and changes the voltage applied to the input terminals of the bridge to the next lower increment.
 12. The apparatus of claim 11 wherein the predetermined level of the first threshold detector is slightly greater than the saturation voltage of the active proportionally controlled transistor and the predetermined level of the second threshold detector is greater than the difference between adjacent incremental voltage levels applied to the bridge circuit. 